1. Field of Art
This invention relates to helicopter controls, and more particularly to utilization of the yaw trim controls of a helicopter to establish force feel to the pedals, thereby to provide the pilot with a cue to tail rotor loading from side slip commands.
2. Description of the Prior Art
In primitive aircraft control systems, including those of a helicopter, the controls manipulated by the pilot were mechanically linked directly to the aerodynamic surfaces being controlled thereby. Aerodynamic resistance to increased aerodynamic loading of these surfaces provided a feel to the pilot of the demands which he is making on the aircraft. However, use of servo systems to assist the pilot in the movement of control surfaces isolates the pilot's feel in the controls from the aerodynamic resistance to increased loading of the surfaces. Therefore, it has been known to provide artificial feel force into the controls of helicopters and other aircraft. Examples of artificial feel force appear in O'Connor and Tefft U.S. Pat. No. 3,733,039 and in Johnson U.S. Pat. No. 4,078,749.
The yaw channel of a modern helicopter controls movement about the yaw axis by means of the tail rotor blade pitch (angle of incidence). This is physically effected by means of a pitch change beam mechanically connected to a yaw servo which provides mechanical force amplification to inputs to the servo from the linkage connected to the pedals. Such a servo, as is known, has a main piston to operate the tail rotor blade pitch beam, to adjust the pitch of the tail rotor blades.
In helicopters having an automatic flight control system (AFCS), the servo has a trim function for automatic pilot control over the yaw channel. In such case, the pedal linkage is mechanically connected, through a resilient means such as springs, to a trim piston. The trim piston is hydraulically controlled by an electromagnetic trim valve, the electric input of which is a function of autopilot control system outputs. The trim valve is in turn rendered operative by opening a trim turn on valve to supply hydraulic fluid under pressure to the trim valve. On the other hand, when the trim valve is disengaged by shutting off the hydraulic fluid to it, the piston is free to move within its hydraulic fluid environment by means of a small bypass orifice, whereby the piston only provides damping to the pedals. The pedal linkage controls the main piston of the yaw servo, whether this linkage is adjusted by force on the pedals or by hydraulic action against the trim piston in response to autopilot commands. When the trim valve is engaged but there is no electric input to it, the trim piston assumes a neutral position. At any position of the piston (a neutral position when trim is on but no autopilot input is provided to it, or the trim position caused by the autopilot function), the pedal linkage may move against the force of springs by which the pedals are mechanically connected to the trim piston. When there is no force on the pedals (feet off), an electric signal to the electromagnetic trim valve will hydraulically adjust the position of the trim piston, which will in turn alter the pedal linkage position and thereby move the pedals accordingly. If the pedal linkage is held in its rest position by the pilot, the trim piston can move, in response to autopilot signals, against the resilient connection to the pedal linkage. But the linkage does not move when the pedals are held at rest; so the pilot can override the autopilot, by holding the pedals at rest.
An example of a yaw autopilot channel, which provides the electrical input to the yaw trim valve, is described in Barnum U.S. Pat. No. 4,067,517. In an AFCS system, when the yaw trim function is selectively engaged by the pilot operating a yaw trim engage switch, the system will automatically provide heading hold control through the yaw trim valve at any speed (including hover) during straight flight. At speeds below cruise speed (such as below 60 knots), the inducement of a turn is sensed to block the heading hold signal, so that the heading hold function will provide no input to the yaw trim valve, and the trim piston will be at a neutral position; the pilot may control the turn (especially at low speeds and in hover) by the pedals. At cruise speeds, the inducement of a turn is sensed to engage a coordinated turn autopilot input to the yaw trim valve, so that yaw continues to be controlled automatically, but is governed to provide the necessary degree of side slip to coordinate a turn at the airspeed of the helicopter, with the amount of roll which the pilot has commanded. Provision of the automatic change-over between the heading hold autopilot function and the coordinated turn autopilot function at cruise speeds is disclosed in Adams and Johnson U.S. Pat. No. 4,003,532.
To avoid the possibility of the pilot fighting the autopilot system, by trying to induce a heading change with the yaw pedals at low speed, while the heading hold system tries to maintain the same heading automatically, or by trying to coordinate a rolled-turn at cruise speeds, switches have been provided in series with the yaw trim engage switch, which open when the related pedal is moved from the rest position. Therefore, any attempt by the pilot to control yaw by means of the pedals has resulted in closing the yaw trim turn on valve to thereby disengage the yaw trim function.